^I just don't buy it. As I said, in that context, humanity would certainly know how many violent and dangerous aliens are out there, and the naive optimism of Archer's crew when they started out would make no sense.

I disagree. Just because humanity came into contact with the violent, carnivorous Kzinti early on doesn't mean they would automatically assume all or even most aliens were dangerous. Also, "boomers" came into contact with several hostile alien species and surely some reports reached the general population of Earth so I don't see the problem you do. Not only that, Enterprise had weapons and an armory officer so humanity wasn't totally na´ve from the time they launched (despite not having the phase cannons online because of leaving Earth early).